feat: board post + channel API routes

Implements Phase 2a of board classifieds redesign:

- GET/POST /api/board/posts (list with tag/author filters, create)
- PATCH/DELETE /api/board/posts/:id (author-only)
- GET /api/board/channels (member)
- POST /api/admin/board-channels (admin)
- PATCH/DELETE /api/admin/board-channels/:id (admin)

Adds board_post_created activity type.

server/api/board/posts.get.js

@@ -0,0 +1,24 @@
+import BoardPost from '../../models/boardPost.js'
+import { requireAuth } from '../../utils/auth.js'
+
+export default defineEventHandler(async (event) => {
+  const member = await requireAuth(event)
+
+  const query = getQuery(event)
+  const dbQuery = {}
+
+  if (query.tag) {
+    dbQuery.tags = query.tag
+  }
+
+  if (query.author) {
+    dbQuery.author = query.author === 'me' ? member._id : query.author
+  }
+
+  const posts = await BoardPost.find(dbQuery)
+    .sort({ createdAt: -1 })
+    .populate('author', 'name avatar circle board.slackHandle')
+    .lean()
+
+  return { posts }
+})